Cells draw in most of their nutrients and toxins through receptor-mediated endocytosis, a process which results in sorting receptor-bound cargo into individual clathrin coated vesicles that pinch off from the plasma membrane. The clathrin protein has an unusual triskelion shape that allows it to form a lattice or basket, causing the membrane curvature during this budding or pinching process. The mechanism of clathrin assembly on the plasma membrane is thought to involve salt bridges, while the dissembly from the free-floating vesicle is regulated by Hsc70. Recently the structures of the proximal leg and terminal domain of clathrin were solved, allowing insights into the possible mechanisms of clathrin assembly. We are studying the mechanism of pH dependence of assembly, using structure-based mutagenesis of residues likely contributing to salt bridges or metal ion liganding; exploring the function of the newly discovered clathrin heavy chain repeat (CHCR) ten helix module which repeats seven times within the clathrin molecule, including through the region of proximal leg-proximal leg interaction in the assembly; and, mapping the interactions between clathrin and the AP-2 adaptor which facilitates assembly at physiological pH by structure-based mutagenesis.